a) Field of the Invention
This invention relates to liquid crystal displays comprising cells of well-defined shape, size and aspect ratio, which are filled with liquid crystals, preferably with a guest dye, and novel processes for their manufacture.
b) Background
A polymer dispersed liquid crystal (PDLC) display usually comprises two transparent plates with electrodes placed opposing each other, separated by using spacers. A thin film of PDLC is enclosed between the two plates. The PDLC film may be up to 200 microns thick, but usually having a thickness of between 2 microns and 50 microns. The cell is hermetically sealed in order to eliminate oxygen and moisture, both of which may chemically attack the liquid crystals. A thorough review of the PDLC technologies can be found in the book xe2x80x9cLiquid Crystal Dispersionsxe2x80x9d by P. S. Drzaic (1995).
A PDLC typically consists of micron-size droplets of a low-molecular-weight nematic liquid crystal dispersed in a polymer binder. The nematic droplets strongly scatter light and the material has a white opaque or translucent appearance (xe2x80x9coff statexe2x80x9d). When a voltage difference is imposed between the two electrodes (xe2x80x9con statexe2x80x9d), the electric field aligns the droplets such that the ordinary refractive index of the liquid crystal nearly matches that of the isotropic polymer matrix, substantially reducing the scattering power of the droplets, and thus allowing light to transmit through. In the on state, the cell thus appears clear or transparent, in the off state it appears opaque.
In a guest-host PDLC display, a dye, particularly a pleochroic or dichroic dye, is added as a guest to the liquid crystal to produce a high color contrast display. For example, because the dye molecules have a property to orientate parallel to the liquid crystal molecules, if a dichroic dye having a bar-shaped structure is added to the liquid crystal, the direction of the dye molecules also changes if the molecular direction of the liquid crystal is changed by applying an electric field on the opposing electrodes. Because this dye is made colored or not depending on the orientation direction, it is possible to switch between a colored state (xe2x80x9coff statexe2x80x9d) and a colorless state (xe2x80x9con statexe2x80x9d) by applying a voltage on the two electrodes. The use of dichroic or pleochroic dyes in guest-host PDLC displays to improve the contrast ratio is well known in the art.
A PDLC display may be transmissive and/or reflective. A transmissive PDLC display has an internal illumination source. Imposing a voltage on the two electrodes allows light to pass through the cell. A typical example of a transmissive PDLC display is a PDLC overhead projector. Reflective PDLC displays typically contain a reflective black or colored filter which becomes visible in the transparent state. Reflective PDLC displays may be found in PDA (personal digital assistant) devices. Transmissive and reflective PDLC displays are particularly attractive because polarizers are eliminated. Polarizers substantially reduce light and decrease brightness of both direct view and projection displays. The absence of polarizers also gives a better viewing angle.
The PDLC displays prepared by prior art processes have many shortcomings. For example, the polymer dispersed liquid crystals typically have droplets of very broad particle size distribution, which results in significant hysteresis, higher operation voltage, poor contrast ratio, undesirable red bleedthrough, and low level of multiplexing. However, the hysteresis of PDLC films must be low to show reproducible gray scales, and low voltage operation and high contrast ratio of the device is essential for most PDA applications. Monodispersed liquid crystal particles in the micron size range have been taught in U.S. Pat. No. 5,835,174, (Clikeman, et al.) U.S. Pat. No. 5,976,405 (Clikeman, et al.), and U.S. Pat. No. 6,037,058 (Clikeman, et al.) to reduce the hysteresis and operation voltage, and improve the level of multiplexity. The contrast ratio of PDLC device prepared from the monodispersed particles remains low for most applications. To improve the contrast ratio without trade off in the thickness of the PDLC film and operation voltage, guest dyes preferably, pleochroic dyes or dichroic dyes are needed. However, the prior art processes do not allow for the precise enclosure of a high concentration of guest dyes in the liquid crystal phase during the manufacturing process, such that only a low concentration of dyes may be encapsulated in the monodispersed polymer particles. Some guest dyes may be left outside of the particles, thereby resulting in an increase in Dmin and a lower contrast ratio.
It is highly desirable to create monodispered liquid crystal domains, which would alleviate the requirement of high operation voltage, allow high contrast ratio and high level of multiplexing, and reduce hysteresis.
The first aspect of the present invention is directed to a liquid crystal (LC) display comprising cells of substantially uniform shape, size and aspect ratio. The cells are filled with LC preferably with guest dye(s).
Another aspect of the invention relates to a novel process for the manufacture of such a LC display.
A further aspect of the invention relates to the preparation of cells of substantially uniform shape, size and aspect ratio. The cells enclose LC preferably with guest dye(s) and are formed from microcups prepared according to the present invention. Briefly, the process for the preparation of the microcups involves embossing a thermoplastic or thermoset precursor layer coated on a conductor film with a pre-patterned male mold, followed by releasing the mold before, during or after the thermoplastic or thermoset precursor layer is hardened by radiation, cooling, solvent evaporation, or other means. Alternatively, the microcups may be formed from imagewise exposure of the conductor film coated with a radiation curable layer followed by removing the unexposed areas after the exposed areas have become hardened.
Solvent-resistant, thermomechanically stable microcups having substantially monodispersed size and shape can be prepared by either one of the aforesaid methods. The size of microcups for most display applications is in the range of submicrons to 10 microns, more preferably 0.5 microns to 5 microns. The shape may be any shape, although a shape allowing a higher total area of interface between liquid crystal and the cups is preferred. The microcups are then filled with LC preferably with guest dye(s), and sealed.
Yet another aspect of the present invention relates to the sealing of the microcups filled with the LC preferable with guest dye(s). Sealing can be accomplished by a variety of ways. Preferably, it is accomplished by dispersing into the LC phase before the filling step, a sealant composition containing a thermoplastic or thermoset precursor. The sealant composition is immiscible with the LC and has a specific gravity lower than that of the LC. After filling, the thermoplastic or thermoset precursor phase separates and forms a supernatant layer at the top of the LC. The sealing of the microcups is then conveniently accomplished by hardening the precursor layer by solvent evaporation, interfacial reaction, moisture, heat, or radiation. UV radiation is the preferred method to seal the microcups, although a combination of two or more curing mechanisms as described above may be used to increase the throughput of sealing. Alternatively, the sealing can be accomplished by overcoating the LC with a sealant composition containing the thermoplastic or thermoset precursor. The solvent used in the sealant composition is critical. Preferably, it is immiscible with the LC and has a specific gravity lower than that of the LC. It is also important to control the surface tension and viscosity of the sealant composition to ensure a good coating uniformity. The sealing is then accomplished by hardening the sealant composition by solvent evaporation, interfacial reaction, moisture, heat, radiation, or a combination of curing mechanisms. These sealing processes are also unique features of the present invention.
Yet another aspect of the present invention relates to the absence of the hysteresis of the LC displays. The LC displays of the present invention consist of substantially monodispersed microcups filled with liquid crystals and preferably a guest dye. The composition of the microcups is optimized such that the isotropic refractive index of the cups is matched to the ordinary refractive index of the LC. In a manner similar to conventional PDLC displays, the LC displays of the present invention strongly scatter light in the absence of an electric field (the xe2x80x9coff statexe2x80x9d). When a voltage difference is applied between the two electrodes, the electric field aligns the LC and substantially reduce scattering power and allow light to transmit through the xe2x80x9con statexe2x80x9d. However, unlike the PDLC displays, the LC displays of this invention reach the maximum optically clear state at a much lower voltage and, when the applied voltage is withdrawn, reverts back to the original xe2x80x9coffxe2x80x9d state without undesirable hysteresis. The low operation voltage, fast response time, and the absence of hysteresis of the displays of the present invention are critical for high quality display applications where low power consumption, reproducible gray scales and video rate are highly desirable.
Yet another aspect of the present invention relates to the manufacture of a LC display comprising stack of layers of microcups. The processing steps include preparation of the first layer of microcups on a conductor substrate by anyone of the methods described above, filling the microcups with a LC composition, sealing, and finally laminating the sealed microcups with a second conductor substrate precoated with an adhesive layer. The adhesives can be hot-melt, heat curable, moisture curable, or radiation curable adhesives. Of particular interest, UV curable adhesives are used in the present invention. To improve the contrast ratio, more than one layer of the microcup array may be used. The processing steps include preparation of microcups, filling with the LC composition, sealing, overcoating or laminating the sealed microcup array with a second layer of the microcup forming composition, forming the second microcup array with any of the methods described previously, preferably by the embossing method, repeating several times the steps of filling and sealing of microcups, overcoating or laminating the sealed microcups with another layer of microcup forming composition, and finally laminating the stack of microcup layers to a second conductor substrate precoated with an adhesive layer.
Yet another aspect of the present invention relates to a process for the manufacture of a full color reflective LC display by using a conductor substrate precoated with (R, G, B) color filers and preferably a black matrix. Alternatively a full color display can be prepared by laminating the preformed microcups with a layer of positively working photoresist, selectively opening a certain number of the microcups by imagewise exposing the positive photoresist, followed by developing the resist, filling the opened microcups with a LC composition containing guest dye(s), preferably dichroic dyes of the first color, and sealing the filled microcups by a sealing process described previously. These steps may be repeated to create sealed microcups filled with LC compositions containing guest dyes of the second or the third color. A black background may be used to improve the contrast ratio and color saturation.
These multiple-step processes as disclosed may be carried out roll-to-roll continuously or semi-continuously. Consequently, they are suitable for high volume and low cost production. These processes are also efficient and inexpensive as compared to other processes for high volume production operations. The LC display prepared according to the present invention is not sensitive to environment, particularly humidity and temperature. The display may be very thin, flexible, durable, easy-to-handle, and format-flexible. Since the LC display prepared according to the present invention comprises cells of favorable aspect ratio and substantially monodispersed shape and size, the displays manufactured according to the present invention exhibit many desirable properties such as low power consumption, fast response time, high level of multiplexing, high contrast ratio, and reproducible gray scale presentation due to the absence of undesirable hysteresis.